1. Field of the Invention
The present invention relates to a polishing device. In particular, the present invention relates to a polishing device for use in a chemical-mechanical-polishing (CMP) process in manufacturing a semiconductor device.
2. Description of the Related Art
In recent years, since the density for integration of a semiconductor device has been increased, the number of interconnection layers in a multilayer interconnection structure is increased in the semiconductor device. In a photolithographic process for the semiconductor device having the multilayer interconnection structure, the uniformity of the film surface exposed by an exposure system should correspond to the range of focal depth of the light source used in the exposure system. Thus, the technique for planarization of the film surface has become more and more important. A CMP process is adopted as one of the planarization processes.
FIG. 7 shows an example of a polishing device used in the CMP process. The polishing device, generally designated at numeral 100, includes a polishing pad 11 having a circular polishing surface 11a for polishing the main surface of a target wafer 12. A polishing head 13 is disposed to oppose the polishing surface 11a with the wafer 12 being interposed therebetween. The polishing head 13 includes a head body 14 having a disc-like shape, a membrane sheet 17 disposed on the head body 14 to be in contact with the rear surface of the wafer 12, and a retainer ring 16 disposed to encircle the membrane sheet 17 and wafer 12 for retention of the same. A closed space 18 is formed between the bottom surface of the head body 14 and the membrane sheet 17. High-pressure air is supplied into the closed space 18 to thereby expand the membrane sheet 17, which presses the wafer 12 against the polishing pad 11.
The top surface of the retainer ring 16 is fixed onto the bottom surface of the head body 14, whereas the bottom surface of the retainer ring 16 abuts the polishing surface 11a of the polishing pad 11. FIG. 8 is a top plan view showing the positional relationship between the retainer ring 16 and the wafer 12. The retainer ring 16 is of an annular shape, and the inner edge of the retainer ring 16 abuts the periphery of the wafer 12 to retain the wafer 12 in the in-plane direction thereof.
During polishing the wafer 12, the polishing pad 11 is rotated about the center thereof, while slurry (abrasive) is supplied onto the central position of the polishing surface 11a. The membrane sheet 17 presses the rear surface of the wafer 12 retained inside the retainer ring 16, whereby the main surface of the wafer 12 is pressed against the polishing surface 11a. In this state, the polishing head 13 is rotated about the central axis thereof and reciprocated in the radial direction of the polishing surface 11a. Thus, the CMP processing for the main surface of the wafer 12 is carried out.
Patent Publications JP-2004-119495A and JP-2002-367941A, for example, describe a CMP apparatus such as the polishing device shown in FIG. 7. The inventor analyzed the function and defect of the conventional polishing device 100, as detailed below.
In the polishing device 100 shown in FIG. 7, the periphery of the membrane sheet 17 is substantially aligned with the periphery of the wafer 12. In this structure, when the high-pressure air is supplied into the closed space 18 for polishing the wafer 12, the membrane sheet 17 is deformed to have a convex shape as viewed in the downward direction. FIG. 9 shows the structure of the peripheral portion of the membrane sheet 17 and wafer 12 during the polishing. The vicinity of the periphery of the membrane sheet 17 is slightly lifted away from the rear surface of the wafer 12, thereby preventing a sufficient pressure from being applied onto the periphery of the wafer 12. The insufficient downward pressure in the peripheral area leads to lowering of the polishing rate and causing an insufficient within-wafer uniformity in the wafer 12 after the polishing process, thereby degrading the product yield of the resultant semiconductor devices.